The present invention relates to a method of controlling the performance of a vehicle in which the forces acting upon wheels and tires are detected by wheel force and tire sensors and are used as controlled variable(s) for an automotive servo-system, such as ABS, TCS, EHB, EMB etc. The variables are referred to in order to determine and/or modulate the brake pressure in the wheel brakes of the wheels and/or the driving torque.
A lot of similar methods controlling the performance of a motor vehicle are known which use the tire sensors for detecting the forces and moments acting upon the tires. While in EP 04 441 09 B1 the deformation of the tread area of the tirexe2x80x94the tread contact areaxe2x80x94is monitored, in WO 96/10505 the deformation of the side wallxe2x80x94the torsion deformationsxe2x80x94of a tire is detected by measuring a space of time between the passing of at least two marks on the rotating wheel which are located on different radii to the rotation axis. WO 97/44673 describes a tire sensor detecting a change of the phase position between measuring signals emitted by measuring elements due to forces acting upon the wheel in case of deformation and evaluating said change of the phase position as dimension for the moments and/or the actual coefficient of friction. In automotive servo-systems the forces acting upon the tire in such a way and detected by the tire sensor are referred to so as to determine and/or modulate the brake pressure in the wheel brakes of the wheels.
Apart from these automotive servo-systems which control the determination and/or modulation of the brake pressure in the wheel brakes of the wheel on the basis of forces detected with tire sensors and/or moments between the tire and the roadway, automotive servo-systems, such as ABS and/or TCS are referenced which are equipped with conventional sensor devices for detecting the four wheel speeds of a motor vehicle. If a wheel during braking or accelerating exceeds the optimal slippage area leading to the risk that the wheels block or spin, the ABS or TCS control automatically intervenes.
With said referenced automotive servo-systems, such as ABS or TCS, the information needed for the control is obtained by determining the rotational performance of the single wheels, where a vehicle reference speed is determined by logical linkage of the wheel rotation signals which approximately reflects a motor vehicle speed. Said vehicle reference speed is then referred to determine the wheel slippage and other controlled variables and finally to control the brake pressure in the wheel brakes of the wheels. Known ABS or TCS servo-systems determine the wheel slippage up to a slip limit value, i.e. the critical slip value, up to which the transmittable brake force or the drive torque only increases, by braking or driving the vehicle in excess of said critical slip, whereupon the circumferential wheel speed changes with regard to the vehicle reference speed in such a way that the tendency of the wheels to block or spin is detected. The control cycles consisting of the phases which appear in any sequence: maintain pressure, build up pressure, reduce pressure, can be repeated several times per wheel in presence of an ABS or TCS servo-system.
Since the critical slip has to be exceeded in order to recognize the slip limit value, the braking or driving performance in each ABS or TCS control cycle is reduced. At the same time the stopping distance is increased. By exceeding the critical slip the lateral forces on the wheel are reduced. The vehicle stability and the maneuverability of the vehicle is reduced.
It is therefore desirable to have a method of controlling the performance of a motor vehicle in which sensors detect or determine the slip limit value or a slip value on the wheel suitable for ABS or TCS control without having to exceed the slip limit value or critical slip value for detecting or determining it.
Tire sensors detecting the forces transmitted by the wheel onto the roadway and acting upon the tire may in principle be used as controlled variables in an ABS or TCS servo-system since they detect the forces immediately at the point where they develop. But since there are xcexc-slip curves in which the coefficient of friction (xcexc) increases in proportion to the slippage thus increasing the force between tires and roadway in case of blocking wheels, the slip values suitable for ABS or TCS control cannot be determined clearly by means of the forces detected with the tire sensors.
It is an object of the present invention to create a method of controlling the performance of a vehicle which allows to optimize the influence on the movement of the vehicle.
The particularity of the method consists in that the working point and/or operative range of forces detected by means of wheel force or tire sensors is adjusted by the detected wheel slippage at the beginning of an ABS or TCS control action. The wheel slippage is determined subject to values measured and/or calculated with conventional sensors, e.g. wheel speeds and motor vehicle reference speeds, and the evaluation of the dynamic rotational performance of the wheel. The working point and/or the operative range of the controlled variables detected with wheel force or tire sensors is adjusted by the simultaneous determination of the forces at the critical wheel slippage and/or at least one wheel slippage and at the wheel reversing point of the speed or its derivation in the first ABS or TCS control cycle. The maximum force is determined when the brake or drive slip or the negative (ABS) or positive (TCS) acceleration begins to increase, i.e. within the range of the critical slip value during the first ABS or TCS control cycle. Furthermore the minimum force is determined in case of a positive acceleration from the brake slip or a negative acceleration from the propulsive slip and re-acceleration from the wheel slippage. The working point and/or operative range of the controlled value xe2x80x9cforcexe2x80x9d is adjusted by balancing the evaluated rotational performance of the single wheels and the vehicle reference speed during the critical slip with the forces determined by the wheel force or tire sensors, especially the longitudinal forces. Thus the present invention is based on the finding that the working point and/or operative range of the forces detected by means of wheel force or tire sensors is recognized at the beginning of the first control cycle by detecting the wheel slippage, i.e. the critical slippage, by means of the speed performance and the vehicle reference speed in order to determine definitely the frictional connection between tire and roadway and exclude errors of the controlled variables basing on xcexc-slip curves in which the coefficient of friction (xcexc) and thus the force between the tires and the roadway increases in proportion to the slip in case of blocking or spinning wheels.
The method according to the present invention allows that the working point and/or operative range of the forces acting upon the vehicle wheel is adjusted within the ABS or TCS control range of the wheel slippage. Therefore the rotational performance of the wheels in the unstable area is detected for the first time at the beginning of the control process and evaluated, if necessary, by including further variables. Afterwards the subsequent ABS or TCS control cycles, i.e. the control of the wheel slippage, are adjusted below the critical slip value up to the critical slip (slip limit value) by means of the forces detected with wheel force or tire sensors, as long as no external influences, as for example, changes of the coefficient of friction, shift the working point and/or the operative range and the slip limit value is exceeded.
According to a particularly advantageous embodiment of the present invention, the determination of the working point and/or operative range described above begins from the start so that the working point and/or operative range is adjusted according to the new parameters of condition variables.
It is a further advantage of the present invention that the critical wheel slippage and/or at least one wheel slippage in the ABS or TCS control range is determined on the basis of the values measured with conventional sensors, simultaneously determining the forces by means of wheel brake or tire sensors, because at the beginning the ABS or TCS control the working point or operative range of the controlled variable(s)xe2x80x94forcexe2x80x94can be determined easily by means of conventional control methods.
One embodiment foresees that in a first ABS or TCS control cycle a force is determined on the one hand in case of increasing brake or drive slip and/or an acceleration of the wheels and on the other hand at a wheel reversing point of the speed or its derivation, i.e., in case of a re-acceleration from the brake slippage or a deceleration from the propulsive slippage of the wheels, because the forces in case of increasing brake or drive slip or an acceleration of the wheels not only depend on the slip and/or the deceleration but also on the increase of force which is influenced by different parameters, as for example a lower pressure of the wheels or wheels with higher slippage need.
Since the forces are determined preferably at the beginning of the brake or drive slip increase and/or acceleration of the first ABS or TCS control cycle, the controlled variables determined by means of the wheel force or tire sensors can be referred to for ABS or TCS control immediately after having determined the forces, so that said controlled variables determined by means of the sensors can be controlled within a stable wheel slippage range, thus avoiding a reduction of the lateral force existing in the unstable wheel slippage range, the impairments of the vehicle steering connected with this as well as the reduction of the brake performance.
In another embodiment of the present invention the brake pressure or the drive torque is controlled according to the relation
F=k1xc3x97Fmax
with Fmax=longitudinal force in the critical slip range
k1=proportionality factor
F=actual longitudinal force
on entering the area of driving stability. In order to eliminate vibrations of the chassis, the longitudinal force is adjusted in a stepped manner. After having determined the first working point as described above, reduced the wheel slippage with the controlled variable Fmaxxc3x97k1 and adjusted the brake pressure corresponding to the force of the basic drive torque, the force and the slippage and the acceleration on the wheel are observed. If the product Fmaxxc3x97k1 is bigger than the forces F detected with the wheel force or tire sensors, the ABS or TCS control is concluded prematurely according to the relation:       T          A      ⁢              xe2x80x83            ⁢      B      ⁢              xe2x80x83            ⁢      S      ⁢              xe2x80x83            ⁢      TCS        =            T      0                                k          1                xc3x97                  F          max                    -      F      
T0 being a theoretical time between 60 and 90 ms, preferably 70-80 ms and TABS/TCS being the exiting time.
By building the product Fmaxxc3x97k1 and the evaluation, i.e. if Fmaxxc3x97k1 is bigger than the forces F detected by means of the wheel force or tire sensors, an increasing coefficient of friction between tire and roadway can be assumed with constant brake pressure introduction or drive torque preset by the driver. In this case the ABS or TCS mode is left prematurely with an exiting time which is indirectly proportional to the difference k1xc3x97Fmaxxe2x88x92F because a vehicle performance with decreasing blocking or drive slip tendency can be assumed.
If the evaluated product Fmaxxc3x97k1 corresponds approximately to the force F measured with the wheel force or tire sensors, a braking or acceleration with homogeneous coefficient of friction can be assumed. The evaluation of k1xc3x97Fmax depends on the exact determination of the force Fmax determined at increasing brake or drive slip or during the acceleration of the wheels, and thus the braking performance in case of ABS control and the performance of the drive and the acceleration in case of TCS control.
By means of self-testing cycles the forces Fmax and Fmin are continuously updated and corrected, if necessary. For this reason the brake pressure is modulated or built up after a certain space of time by means of a pressure build-up impulse which is introduced into the servo-system and/or by means of a added drive torque introduced into the TCS control cycle, where the positive or negative acceleration of the wheels and the forces is determined. The evaluation of the change of the measured forces and the acceleration leads to the assumption that the brake or drive force is insufficient, e.g. if the forces F increase and the acceleration remains unchanged, i.e. if the forces F increase without an increased acceleration being determined. In other words: the forces F measured and evaluated with the tire sensors, in particular the longitudinal forces detected between tires and roadway, do not correspond to the forces which can be transmitted from the wheel or wheels onto the roadway. In this case the working point or operative range must be found once again.
According to one embodiment of the present invention the force Fmax and/or the acceleration is evaluated by comparison with a threshold value (S) and an instability and thus a brake slip or a drive slip is caused when the threshold value is exceeded by the modulation or the build-up of brake pressure or the increase of the drive torque, so that a new adjustment of the working point by means of the wheel slippage is caused as described above. New adjustment or new determination of the working point is preferably begun by means of the wheel slippage in case of high vehicle speeds or high forces.
According to another embodiment of the present invention, the maximum force Fmax or the acceleration is evaluated by means of comparison and the measured force F is approached to the maximum force (Fmax) by means of a modulation or the build-up of the brake pressure by introducing at least one pressure build-up impulse, usually several pressure build-up impulses and/or a drive torque added into the control system, if the threshold value (S) is not reached, in particular in case of Fmax or a(acceleration) less than S. In this case the wheel being considered presents a small extent of slippage, i.e. the critical slip value is exceeded and a new working point is adjusted. Control is foreseen in an advantageous manner in case of low force or low vehicle acceleration.
A further embodiment of the method foresees that in case of an approximately constant or non increasing force F and an increase of the positive or negative acceleration or the measured force according to the relation F greater than k1xc3x97Fmax the wheel or wheels are within the ABS or TCS control range of the wheel slippage. In this case a drive torque or brake pressure reduction is introduced by means of a brake pressure reduction impulse of approx. Tp=2 ms or an added drive torque until Fxe2x89xa6k2 * Fmin with k2 greater than 1.
Additional objects and features of the present invention will be readily apparent to those skilled in the art from a review of the attached drawings, the following detailed description and the appended claims.